In inkjet printing, minute droplets of ink are ejected in a controlled manner from one or more printing heads through narrow nozzles on to a substrate which is moving relative to the printing head(s). The ejected ink forms an image on the substrate. For high-speed printing, the inks must flow rapidly from the printing heads, and, to ensure that this happens, they must have in use a low viscosity, typically 200 mPas or less at 25° C., although in most applications the viscosity should be 50 mPas or less, and often 25 mPas or less. Typically, when ejected through the nozzles, the ink has a viscosity of less than 25 mPas, preferably 5-15 mPas and ideally 10.5 mPas at the jetting temperature which is often elevated to 40-50° C. (the ink might have a much higher viscosity at ambient temperature). The inks must also be resistant to drying or crusting in the reservoirs or nozzles. For these reasons, inkjet inks for application at or near ambient temperatures are commonly formulated to contain a large proportion of a mobile liquid vehicle or solvent such as water or a low-boiling solvent or mixture of solvents.
Another type of inkjet ink contains unsaturated organic compounds, termed monomers and/or oligomers which polymerise by irradiation, commonly with ultraviolet light, in the presence of a photoinitiator. This type of ink has the advantage that it is not necessary to evaporate the liquid phase to dry the print; instead the print is exposed to radiation to cure or harden it, a process which is more rapid than evaporation of solvent at moderate temperatures. In such inkjet inks it is necessary to use monomers possessing a low viscosity.
In inkjet printing, the inkjet printhead moves relative to the substrate from one end of the substrate to another laying down the ink on the substrate as it passes over the substrate. This movement of the printhead relative to the substrate is termed a “single pass” of the inkjet head relative to the substrate. The ink that is applied to the substrate during this single pass may form the whole image or a part of the image. Where the whole image is formed, no further passes of the printhead over the substrate are required and the technique is termed “single-pass” printing. Where only part of the image is formed, further passes of the printhead are required and the technique is termed “multi-pass” printing. Thus, multi-pass mode occurs when not all of the ink required for the complete image on a substrate is applied during one pass of the printhead over the substrate.
In multi-pass printing, the odd and even numbered passes may be in the same or opposite directions. The former is termed “uni-directional” multi-pass printing and the latter is termed “bi-directional” multi-pass printing.
Printers for multi-pass printing typically have a range of flatbed printing platforms which use printheads to deposit ink droplets over a series of print passes to build an image. An example is the Inca Onset S40i, as shown in FIG. 1. In FIG. 1, the following reference numerals apply: printing bed with substrate (1), a first and second UV lamp (2,3), printhead (4), a power map for recording UV dose and intensity value (5) and the printing direction (6).
As shown in FIG. 1, the printheads are configured across the whole width of the print area, and the substrate moves to-and-fro under the printheads. FIG. 1 shows the printhead printing a first layer of ink in a single pass. The left-hand side of the figure shows the substrate on a flatbed prior to a pass and the right-hand side of the figure shows the substrate after a first pass. The substrate will then move back to its original position laying down another layer of ink in a second pass. Optionally the printhead may index (i.e. they shift slightly left or right based on the direction of travel) between passes. A printhead has an array of nozzles, usually a linear array, typically from 128 to 2,056, with 256 nozzles being common. The indexing of the printhead ensures that if any nozzles in the printhead are not firing, the absence of ink on a single given pass is not noticeable in the final image.
As each portion of the total amount of ink is applied to the substrate on each pass of the printhead, the ink is cured by exposure to actinic radiation. The radiation source (typically referred to as a “lamp”) may be integral to the printhead or separate, but usually integral. The printhead usually has a leading and a trailing lamp for optimising the cure.
Printers of this type usually have two print modes as standard. A “satin mode” is the highest productivity mode, and the number of print passes is selected based on the quality of image required. There are no additional cure passes associated with this print mode. UV lamps are adjusted to a suitable power to ensure good ink cure and effective substrate heat management. A “gloss mode” is a lower productivity mode and is used when a higher gloss or gamut is required. The lamp is shuttered or the power reduced for all print passes to give a low “pinning” UV dose, which can be varied to achieve the required level of gloss. To achieve full cure, two additional slow high UV hard cure passes are applied to the print. FIG. 2 shows the relationship between colour development and gloss.
In “gloss mode”, it is not always possible to achieve good adhesion or productivity. Another method is to employ a time delay between print and cure to allow the ink to flow further before curing (see WO 2012/110815). This often results in poor reproduction of fine text or other print quality issues such as bleeding and mottling.
Another approach has been to formulate inks for slower curing and/or employing inherently glossy raw materials. However, as productivity and performance requirements require faster or higher functionality inks, using faster curing or lower gloss raw materials leads to dot gain reduction and gamut reduction in traditional hard cure “satin print” modes.
There remains a need in the art for widening the ink formulation latitude, whilst still providing high performing inks with acceptable gamut, gloss and print quality.